In known approaches, as are freely available on the market, filter devices are used in all types of hydraulic systems, especially in construction machines. Filter units, called return line intake filters, are in the return lines of the respective hydraulic system discharge. The return lines are combined into collectors which ensure central inflow of the fouled fluid or hydraulic medium to the respective return line intake filter unit. After filtration, the cleaned fluid is relayed to an intake connection to supply the hydraulic system with the cleaned hydraulic medium.
To prevent overheating of the hydraulic system, a cooling unit can be connected upstream to the supply line into which the return lines discharge before admission into the actual filter circuit in the secondary branch line. A conventional thermostat controller can turn on or off the cooling unit depending on the operating temperature of the hydraulic medium. Especially at low temperatures of the hydraulic medium, the changeover means, with the thermostat controller and/or the components of the cooling circuit itself can produce a banking-up pressure which leads to pressure peaks in the upstream units. For example, in a hydraulic motor and a hydraulic pump pressure peaks can damage the sealing elements. Furthermore, the fluid-carrying components in the known approach are connected to one another by the indicated collectors. Lines producing the pertinent fluid-carrying connections, can take place by conventional tubing technology using weld connections. Therefore, the known filter devices are structurally large and expensive to produce.
DE-A-34 02 884 A1 discloses a filter device with a thermostat installed in the system for regulating the temperature of the working liquid and opening or closing automatically the branch to the cooler depending on the temperature of the reversible working liquid in the system. The heated reversible working liquid flows directly out of the thermostat upstream of the filter into the intake branch of the filling liquid pump, i.e., bypassing the cooler and the tank. The thermostat closes the throughflow of working liquid into the part of the circuit which contains the cooler and tank when the temperature of the working liquid is low. When the temperature of the working liquid reaches the value at which the thermostat opens, part of the working liquid flows through the connecting branch into the cooler and on into the tank. The thermostat opens entirely when the temperature of the working liquid in the tank reaches the optimum value. All the working liquid which returns from the closed hydraulic circuit continues to flow only by way of the cooler into the tank. In this known approach, the thermostat can produce a banking-up pressure which leads to damaging pressure peaks in the upstream units especially at low temperatures of the hydraulic medium within the hydraulic circuit.
U.S. Pat. No. 3,990,424 discloses a generic filter device with at least one filter unit connected to a hydraulic circuit with at least one supply and drain line. At least one cooling unit is connected in a secondary branch line and is connected parallel to the supply line into the hydraulic circuit. By a changeover means, at a definable temperature threshold value of the hydraulic medium to be cooled, the medium is supplied to the circuit. In the flow direction of the hydraulic medium in the supply line upstream of the changeover means, a permanent supply for the cooling unit is provided. This permanent supply is connected parallel to the changeover means discharging into the secondary supply line. In this known filter device used especially in jet-propelled racing boats, the permanent supply is induced by the valve chamber to which the cooling unit is permanently connected to carry fluid. Depending on the respective temperature threshold value and therefore depending on the position of the changeover means, at low temperatures of the hydraulic medium, the medium is supplied directly to the filter unit. At high temperatures of the hydraulic medium, a detour takes place by the cooling unit before the medium which has then been cooled is supplied to the filter unit. As a result of the direct connection of the cooling unit to the valve chamber in the system, a damaging banking-up pressure can build up which leads to pressure peaks in the upstream units and in the cooling unit itself. In addition, the cooling unit can be exactly matched as a function block to the intended valve chamber using connection technology This arrangement causes production of the known design to become expensive.